4s gets very hot and when SIM is on, Searching signal only.

Hello, this question may not be new. But I have a 4s that until last Saturday, won't pick up signal (searching only) and it gets very hot in the process (even when it is locked). If I remove the SIM card, it doesn't get hot anymore.

I bought a new SIM card, put it in the phone then it went okay for a few minutes. Then it went back on searching, and very, very hot again.

Any solutions? Thank you in advance. Best regards!